Miniaturized Drift Tube Ion Mobility Spectrometer with Ultra-Fast Polarity Switching

Abstract

Ion mobility spectrometers (IMS) are well suited for detecting trace gases down to levels at ppb v and even ppt v within 1 s of analysis time when using chemical ionization. The measuring principle is based on the separation and detection of the ionized constituents of a sample. Depending on the sample composition, certain ionization sources create both positive and negative analyte ions, but the simultaneous detection of both ion polarities usually requires two drift tubes. Contained within this effort, we present an alternative approach for detecting both ion polarities using one single drift tube that can switch the polarity of the drift tube within 12 ms. This technique allows for generating one positive and one negative ion mobility spectrum, each with a drift time range of 13 ms (minimum reduced ion mobility of K 0 = 0.72 cm 2 V −1 s −1 ), within a total experiment time of 50 ms. Additionally, ions are continuously generated in the ionization region during both the polarity switching and the analysis of one of the polarities, which allows for an effective ionization/reaction time of 25 ms. Comparable to the performance of similar instrument designs we reported previously, the presented device has a high resolving power of R P = 70 with a drift length of 51 mm. The limits of detection are for the monomers between 70 and 370 ppt v and for the dimers between 450 and 800 ppt v for 1 s of averaging for various ketones, methyl salicylate, and chlorinated hydrocarbons. Although this work focuses on applying ultra-fast polarity switching to an existing IMS, the techniques shown here may be applied to other IMS implementations for different applications.

Ultra-Fast Polarity Switching, Non-Radioactive Drift Tube for the Miniaturization of Drift-Time Ion Mobility Spectrometer

“…The purpose of aperture gird in IMS instrument is to prevent the generation of induced current as the ion swarm approaches the detector, so that the voltage stability of aperture grid is of key importance for the quality of signal measured on the detector [24]. Normally a large capacitor C, which capacitance value is between 100 nF and 1 µF, is arranged between the aperture grid and ground, in parallel to the voltage dividing resistor R, to stabilized the potential difference between the aperture grid and the ion detector and hence reduce the noise, as shown in Figure 3a.…”

“…If the switching of polarity takes a few seconds and the device happens to work under the negative mode when the TATP is sampled, the analyte will be gone when the polarity is switched back to positive mode, ending up a false negative result. In another application, when used as a detector for GC the switching also has to be fast as possible, since the elution peak only last for seconds and each measurement must take shorter time than the standard deviation to restore the gaussian peak with high fidelity [24].…”

“…To achieve fast switching of polarity, there are four major considerations for the design of the instrument. The first is to establish uniform electric filed in the drift tube after the change of voltage, which requires minimal capacitance between the electrodes used to define such electric field [24][25][26]. Another benefit for smaller capacitance is the reduction of power consumption, allowing the use of smaller power supply module for high fast switching instrument.…”

“…Another benefit for smaller capacitance is the reduction of power consumption, allowing the use of smaller power supply module for high fast switching instrument. The second is concerning the aperture grid in front of the faraday plate, which plays a key role for the elimination of mirror current effect on the ion detector [24,26]. The potential difference of aperture grid and faraday plate is defined by circuit in between, comprising a resistor and a capacitor connecting in parallel.…”

“…Pershenkov et al proposed to use an independent power supply for the aperture grid capacitor to reduce the discharge time, which requires extra cost and volume of the instrument [26]. Hitzemann et al used a much smaller capacitor (220 pF) for the aperture grid, in combination with field switching ion shutter, to achieve a polarity switching time of 12 ms on a test system based on radioactive ionization source [24]. Although fast switching was achieved in this design, small aperture grid capacitor leads to the problem of noise control and faraday plate on HV end requires more sophisticated design of amplification circuit.…”